What to pack?

We'll be there pretty much the same time as you. Here's a thought: Pack light and easily dried, and if it's cold, buy a souvenir sweatshirt. Plenty of those around. It should be plenty temperate.

What I would worry about would be the chilly/wet combination, though I think your odds of avoiding it are quite good 2 weeks from now in Croatia. I'd pack for warm weather but include something like a fleece outer layer and a rain jacket. I take those on every trip, because it can be quite chilly early in the morning and at night, especially if it's windy (as coastal cities often are).

I'm cold-natured, so I would also include a pair of longjohn bottoms, either polypropylene or merino wool. They've helped keep me more comfortable in wet conditions and early in the morning. I wore mine as recently as about a week ago in eastern Spain.

I would not describe spring in Croatia as muggy, especially not away from the coast. Europe has been unseasonably cold this Spring. I wear what I wear at home - jeans, long sleeve cotton shirts, a light jacket. Evenings will be cool. Not the time of year for lightweight anything.

Usually it would be hot with pleasant evenings, but monitor the 10 day forecasts up until you leave. As someone else mentioned, the region is experiencing an unusually cool late spring.